1. Field of the Invention
The present invention relates to an interferometer apparatus which can always perform stable measurement of, e.g., the movement or displacement amount of an object even when a change in temperature is involved.
2. Related Background Art
A laser interferometer is known as an instrument which can measure the amount of change in the relative distance between first and second points with high precision. When the difference between the optical path length of a first laser beam reflected from the first point and that of a second laser beam reflected from the second point changes, interference fringes or a beat signal also change. The laser interferometer is designed to convert such a change in interference fringes or in beat signal into count pulses. For example, by accumulating these count pulses, the relative distance between the first and second points can be obtained.
In this case, the optical path length of each laser beam is the product of the actual length of the optical path of the laser beam and the refractive index of a medium through which the beam passes. As disclosed in Japanese Laid-Open Patent Application No. 63-228003 for example, an optical path length is generally constituted by a portion based on a large number of optical paths of laser beams which pass through air having a small refractive index (close to 1), and a portion based on a large number of optical paths of laser beams which pass through a medium such as a glass medium having a large refractive index. If, therefore, the refractive index of the glass medium or the like changes with a change in the internal temperature of the medium due to a change in ambient temperature or a thermal change of a support member, the optical path difference changes even if the relative distance between the first and second points remains the same. As a result, a change in distance may be erroneously detected by the laser interferometer.
Recently, the laser interferometer has been used for applications demanding very high measurement precision, e.g., a displacement detecting portion of a projection exposure apparatus for the manufacture of semiconductor elements. It is, therefore, required that measurement errors caused by thermal factors be minimized. In Japanese Laid-Open Patent Application No. 63-228003 described above, the following interferometer is proposed as an interferometer which can reduce measurement errors caused by thermal factors and can detect a change in distance with high precision even at a high temperature.
FIG. 4A shows the conventional interferometer. Referring to FIG. 4A, a two-frequency laser source 1 emits a mixture of first and second beams in the X direction, which beams respectively have frequencies f1 and f2 (f2.noteq.f1) and are linearly polarized in directions parallel and perpendicular to the drawing surface of FIG. 4A. A prism member 2 is formed by bonding a rectangular prism 3 to a parallelepiped prism 4 having parallelograms parallel to the drawing surface of FIG. 4A. The interface between the rectangular prism 3 and the parallelepiped prism 4 serves as a polarization beam splitter surface 2a. One surface, of the parallelepiped prism 4, which is parallel to the surface 2a serves as a reflecting surface 2b. The reflecting surface 2b may be a mirror making use of total reflection. That is, the prism member 2 can be regarded as an optical member having the polarization beam splitter surface 2a and the reflecting surface 2b which are arranged parallel to each other. The prism member 2 is arranged such that the polarization beam splitter surface 2a is perpendicular to the drawing surface of FIG. 4A and crosses the X direction at 45.degree..
The laser beams (first and second beams) emitted from the laser source 1 are incident on the polarization beam splitter surface 2a of the prism member 2 at an incident angle of 45.degree.. Of these beams, the first beam is a p-polarized beam with respect to the surface 2a and is transmitted therethrough to be incident on a reference mirror 6 through a .lambda./4 plate (quarter wave plate) 5. The first beam reflected by the reference mirror 6 is incident on the polarization beam splitter surface 2a of the prism member 2 again through the .lambda./4 plate 5. In this case, since the first beam reciprocates through the .lambda./4 plate 5, the beam becomes an s-polarized beam with respect to the surface 2a. For this reason, the first beam is reflected by the surface 2a to propagate toward a corner-cube prism 7.
The first beam reflected by the corner-cube prism 7 is incident on the polarization beam splitter surface 2a of the prism member 2 after the plane of polarization is rotated through 90.degree. by a .lambda./2 plate (half wave plate) 8. The beam incident position is shifted from the exit position in a direction parallel to the surface of FIG. 4A. In this case, since the first beam is converted into a p-polarized beam by the .lambda./2 plate 8, the beam is transmitted through the surface 2a and is reflected by the reflecting surface 2b. The first beam is then incident on the reference mirror 6 through the .lambda./4 plate 5. The first beam reflected again by the reference mirror 6 passes through the .lambda./4 plate 5 and is reflected by the reflecting surface 2b of the prism member 2 so as to be incident on the polarization beam splitter surface 2a. At this time, since the first beam is converted into an s-polarized beam upon reciprocating through the .lambda./4 plate 5, the beam is reflected by the surface 2a and is incident on a receiver 10. An analyzer and a light-receiving element are incorporated in the receiver 10.
Of the laser beams from the laser source 1, incident on the polarization beam splitter surface 2a of the prism member 2, the second beam is an s-polarized beam with respect to the surface 2a. The second beam is reflected by the surface 2a and is then reflected by the reflecting surface 2b. Thereafter, the second beam is incident on a movable mirror 9 through the .lambda./4 plate 5. The movable mirror 9 is wider than the reference mirror 6, and is held in a region shifted therefrom in the X direction to be movable in the X direction. The second beam reflected by the movable mirror 9 is transmitted through the .lambda./4 plate 5 and is reflected again by the reflecting surface 2b of the prism member 2 to be incident on the polarization beam splitter surface 2a. In this case, since the second beam is converted into a p-polarized beam upon reciprocating through the .lambda./4 plate 5, the beam is transmitted through the polarization beam splitter surface 2a to propagate toward the corner-cube prism 7.
The second beam reflected by the corner-cube prism is incident on the polarization beam splitter surface a of the prism member 2 after the plane of polarization is rotated through 90.degree. by the .lambda./2 plate 8. In this case, since the second beam is converted into an s-polarized beam by the .lambda./2 plate 8, the beam is reflected by the surface 2a to be incident on the movable mirror 9 through the .lambda./4 plate 5. The second beam reflected again by the movable mirror 9 is incident on the polarization beam splitter surface 2aof the prism member 2 through the .lambda./4 plate 5. At this time, since the second beam is converted into a p-polarized beam upon reciprocating through the .lambda./4 plate 5, the beam is transmitted through the surface 2a and is incident on the receiver 10. In the receiver 10, the polarization directions of the first and second beams respectively reflected twice by the reference mirror 6 and the movable mirror 9 are aligned by the analyzer so that the first and second beams are incident on the light-receiving element.
While the movable mirror 9 is at rest, a beat signal having a frequency (f1-f2) is output from the light-receiving element of the receiver 10. When the movable mirror 9 moves, a frequency-modulated beat signal is output. Therefore, by accumulating such changes in frequency, the movement amount of the movable mirror 9 in the X direction with respect to the reference mirror 6 can be detected.
The optical paths of the first and second beams in the prism member 2 of the interferometer in FIG. 4A will be considered with reference to FIG. 4B. As shown in FIG. 4B, the first and second beams have optical paths T11 and T21, respectively, within the parallelepiped prism 4 of the prism member 2, excluding the common optical path. Within the rectangular prism 3, the first and second beams have optical paths T12 and T22, excluding the common path. Since T11+T12=T21+T22, the optical path lengths of the first and second beams are equal in the prism member 2 as a whole.
When, however, considering the parallelepiped prism 4 and the rectangular prism 3 separately, T11&gt;T21 is established in the parallelepiped prism 4; and T12&lt;T22, in the rectangular prism 3. Therefore, even if the parallelepiped prism 4 and the rectangular prism 3 have the same refractive index, the optical path length difference between the first and second beams changes if the temperatures of the two prisms differ from each other. As a result, in spite of the fact that the movable mirror 9 is at rest, this change is erroneously detected as a change in the movement amount of the mirror 9. That is, with the arrangement shown in FIG. 4A, when a temperature difference occurs between the optical members constituting the prism member 2, a measurement error is caused.
The present invention has been made in consideration of the above situation, and has as its object to provide an interferometer apparatus which can achieve a reduction in measurement error even when a temperature difference is present between optical members.